1. Field of the Invention
The present invention relates to an improvement in a disc brake for a vehicle.
2. Prior Art
Pads which are employed in disc brakes must endure high surface pressure, heavy loads and high temperatures, and are therefore formed by using materials that have high wear resistance and good heat resistance.
Accordingly, it has heretofore been common practice to employ asbestos-resin molded materials which are formed by employing asbestos as a basic material and mixing it with a binder resin and various kinds of friction adjusting materials. However, semimetallic and metallic materials utilizing metal wires as basic materials have recently been employed with a view to reducing dust pollution and lengthening the life of such pads.
Metallic pads formed by using semimetallic or metallic materials as basic materials suffer, however, from the following problems. Since the associated disc is also formed from a metallic material, when the disc and the pad come into contact with each other, sparks are generated. If these sparks adhere to the piston boot of the disc brake, the piston boot may be burnt and degraded, which will result in breakage.
In view of the above-described problems, proposals have been made to provide a disc brake in which a shim which is interposed between the backing plate of an inner pad and a piston is extended radially of the disc to form a shielding plate for the purpose of shielding the boot from sparks generated during the contact between the disc and the pads, which type of disc brake is disclosed in Japanese Utility Model Laid Open No. 63-49032 which corresponds to U.S. Pat. Ser. No. 082,357 filed on Aug. , 1987 now U.S. Pat. No. 4,836,339.
The prior art disc brake described above suffers, however, from the following problems.
The prior art disc brake having such a shielding plate must be designed with a clearance of a certain width between the shielding plate and the caliper. Namely, the shielding plate is displaced axially of the disc as the lining of the pad wears. In cases where the caliper is provided with portions having a drag or with stepped portions, therefore, the clearance mentioned above needs to be defined in such a way as to prevent the shielding plate from interfering with those portions when the shielding plate is displaced. Further, calipers are generally made by casting and it is difficult to make calipers with sufficient dimensional accuracy. Consequently it is also necessary to design the caliper and shielding plate with a clearance of a certain width defined therebetween in order to avoid the possibility of proper assembly thereof not being feasible.
In the prior art disc brake with a clearance between the caliper and the shielding plate mentioned above, sparks can pass through the clearance defined between the shielding plate and the caliper and fly toward the piston boot, so the shielding effect is insufficient.
Further there is a problem in that, when the brake is operated with a heavy load, namely, when the pad is pressed onto the disc with a great force, metal powder may be scraped off from the pad and catch fire, and the flames may enter the piston boot side over the shielding plate and hence burn the piston boot.